In many applications a power converter is required to provide a voltage within a predetermined range formed from a voltage source having a different voltage level. Some circuits are subject to uncertain and undesirable functioning and even irreparable damage if supplied power falls outside a certain range. More specifically, in some applications, a precise amount of power is required at known times. This is referred to as regulated power supply. In order to control a power converter to deliver a precise amount of power as conditions require, some form of control of the power converter is required. This control can occur on the primary side of an isolation transformer or the secondary side. A closed loop feedback control system is a system that monitors some element in the circuit, such as the circuit output voltage, and its tendency to change, and regulates that element at a substantially constant value. Control on the secondary side of a power converter can use a monitored output voltage as feedback control, but requires the use of some communication from the secondary to the primary side of the isolation transformer to control the primary side switching element. Control on the primary side can readily control the primary side switching element, but requires some feedback mechanism from the secondary side to the primary side to convey the status of the monitored element. In some applications, an optical coupler circuit, or opto coupler, is used to transmit feedback signals while maintaining electrical isolation between the primary and secondary sides.
FIG. 1 illustrates a conventional regulated switch mode power converter including an optical coupler circuit. The power converter 2 is configured as a traditional flyback type converter. The power converter 2 includes an isolation transformer 4 having a primary winding P1 and a secondary winding S1. The primary winding P1 is electrically coupled to an input voltage Vin and a driving circuit including a transistor 8, a resistor 12, and a controller 10. A capacitor 28 is coupled across the input Vin and coupled with the primary winding P1. Input voltage to the circuit may be unregulated DC voltage derived from an AC supply after rectification and filtering. The transistor 8 is a fast-switching device, such as a MOSFET, the switching of which is controlled by the fast dynamic controller 10 to maintain a desired output voltage Vout. The controller 10 is coupled to the gate of the transistor 8. As is well known, the DC/DC conversion from the primary winding P1 to the secondary winding S1 is determined by the duty cycle of the PWM switching signal provided to the transistor 8. The secondary winding voltage is rectified and filtered using the diode 6 and the capacitor 22. A sensing circuit and a load 14 are coupled in parallel to the secondary winding S1 via the diode 6. The sensing circuit includes a resistor 16, a resistor 18, and a sensing circuit 20. The sensing circuit 20 senses the output voltage Vout across the load.
In this configuration, the power converter is controlled by driving circuitry on the primary side, and the load coupled to the output is isolated from the control. As such, a monitored output voltage used for voltage regulation is required as feedback from the secondary side to the control on the primary side. The power converter 2 has a voltage regulating circuit that includes the secondary controller 20 and an optical coupler circuit. The optical coupled circuit includes two galvanically isolated components, an optical diode 24 coupled to the secondary controller 20 and an optical transistor 26 coupled to the controller 10. The optical diode 24 provides optical communication with the optical transistor 26 across the isolation barrier formed by the transformer 4. The optical coupler circuit in cooperation with the sensing circuit 20 provides feedback to the controller 10. The controller 10 accordingly adjusts the duty cycle of the transistor 8 to compensate for any variances in an output voltage Vout.
However, the use of an optical coupler circuit in and of itself presents issues. Firstly, the optical coupler circuit adds extra cost. In some applications, the optical coupler circuit can add more cost to the power converter than the isolation transformer. The optical coupler circuit also adds to manufacturing and testing costs. Furthermore, the performance of the optical coupler circuit degrades over time and therefore introduces another potential point of failure in the overall power converter. Also, characteristics of the optical coupler circuit must be accounted for in the overall circuit design. For example, the optical diode component is non-linear and as such a correlation between the optical diode and the optical transistor must be established. The optical coupler circuit also has delays related to the operation of the optical diode and the optical transistor, and the operation of the optical diode requires a well defined DC level. As a result, it is generally desirable to avoid the use of an optical coupler circuit.
A next generation of feedback control does not use optical control circuitry. Instead, the transformer is used to convey real-time feedback signaling from the secondary side to the primary side. In such an application, the transformer includes an auxiliary winding on the primary side that is magnetically coupled to the secondary winding. FIG. 2 illustrates a conventional regulated power converter including a magnetically coupled feedback circuit. The power converter 32 is configured as a traditional flyback type converter. The power converter 32 includes an isolation transformer 34 having a primary winding P1 and a secondary winding S1. The primary winding P1 is electrically coupled to an input voltage Vin and a driving circuit including a transistor 44, a resistor 46, and a controller 42. A capacitor 58 is coupled across the input Vin and coupled with the primary winding P1. Input voltage to the circuit may be unregulated DC voltage derived from an AC supply after rectification and filtering. Similar to the power converter in FIG. 1, the transistor 44 is a fast-switching device controlled by the fast dynamic controller 42 to maintain a desired output voltage Vout. The secondary winding voltage is rectified and filtered using the diode 36 and the capacitor 38, with the output voltage Vout delivered to the load 40.
The power converter 32 has a feedback loop that includes a magnetically coupled feedback circuit coupled to the secondary winding S1 of the transformer 34 and the controller 42. The magnetically coupled feedback circuit includes a diode 48, a capacitor 50, resistors 52 and 54 and an auxiliary winding 56. The auxiliary winding 56 is coupled in parallel to the series of resistors 52 and 54.
The voltage VA is proportional to the voltage across the auxiliary winding 56. The voltage VA is provided as a feedback voltage VFB to the controller 42. The current through the transistor 44 is also provided as feedback current IFB to the controller 42. The controller 42 includes a real-time waveform analyzer that analyzes input feedback signals, such as the feedback voltage VFB and the feedback current IFB.
The auxiliary winding 56 is also magnetically coupled to the secondary winding S1. When the current through the diode 36 is zero, the voltage across the secondary winding S1 is equal to the voltage across the auxiliary winding 56. This relationship provides means for communicating the output voltage Vout as feedback to the primary side of the circuit. The voltage across the auxiliary winding 56 is measured when it is determined that the current through the diode 36 is zero, which provides a measure of the voltage across the secondary winding S1 and therefore the output voltage Vout.
The power converter can operate in a continuous conduction mode (CCM) or a discontinuous conduction mode (DCM). CCM is defined by continuous output current over the entire switching period, such as the switching period of the transistor 44. DCM is defined by discontinuous output current during any portion of the switching period. As described above, the feedback configuration used in FIG. 2 is effective for output voltage regulation when the power converter is operated in DCM, since it is when the output current is zero that an accurate determination of the output voltage Vout can be provided to the primary side controller.
FIG. 3 illustrates exemplary current and voltage diagrams as measured at given points of the power converter 32 of FIG. 2 during DCM operation. The graph 60 represents the switching signal VG, displayed in volts over time, applied to the transistor 44 which originates from the controller 42. The graph 60 is showing approximately a 50% duty cycle PWM driving signal, but it will be understood that any duty cycle could be used. The graph 70 shows the corresponding switch voltage VSW across the transistor 44 according to the switching signal VG. The switch voltage VSW is displayed in volts over time. The switch voltage VSW has peaks, such as the peak 61 and ringing effects such as a result of parasitics in both the transistor 44 and in the rest of the power converter 32. The graph 80 shows the primary current IP as a function of amps over time. The primary current IP is the current that flows through the primary winding P1 when the transistor 44 is on. The graph 90 shows the secondary current IS as a function of amps over time. The secondary current IS is the current that flows through the secondary winding S1 when the transistor 44 is off. At a time t=1, the gate of the transistor 44 is driven and the transistor is turned ON, or conducting. The switch voltage VSW approaches zero and the primary current IP ramps up thereby storing energy in the primary winding P1. At a time t=2, the transistor 44 is turned OFF, or non-conducting. As a result, energy stored in the primary winding P1 is transferred to the secondary winding S1. The secondary current IS begins ramping down as the stored energy is released until it reaches zero at time t=3. After a delay, the cycle begins anew at time t=4. At the time the secondary current IS reaches zero, the output voltage Vout is determined according to a measured voltage across the auxiliary winding 56.
Although using such a power converter in the DCM is effective for low to mid range power applications, using the power converter in DCM is not practical for higher power applications, such as those applications approaching or exceeding 20 watts, because such an architecture is not electrically efficient, is larger in size and is not cost effective. As such, using the power converter in the DCM for those applications having a relatively high average power, and also those applications having a relatively low or mid average power but durations of higher peak power, is not practical.
Operating the power converter of FIG. 2 in the CCM is also ineffective. Since there is non-zero secondary side current while operating in CCM, there are always voltage losses associated with the transforming winding resistance and the secondary side diode voltage drop. If the auxiliary winding voltage Vaux is used as a feedback signal as described above, it is difficult to compensate for these voltage losses and keep the output voltage in regulation when the primary side controller is operating in CCM during peak power demand.